The present invention relates to an apparatus for determining the density of unoccupied electron states of a material located above the Fermi level, as well as for analyzing by ultraviolet fluorescence of a material excited by electrons. This apparatus associated with a photoemission apparatus, makes it possible to determine the electron structure of a material, the value of the forbidden energy band for new semiconductor or insulants, as well as the orbital character of electrons belonging to the conduction band of this material.
In a metal, the Fermi level coincides with the highest energy level effectively occupied by electrons. Thus, in the case of a metallic material, the Fermi level is located in the conduction band, whereas in insulants and semiconductors the Fermi level is located between the conduction band and the valence band, i.e. in the forbidden energy band.
In order to determine the density of the unoccupied electron states located above the Fermi level of a material, a sample of the material is bombarded by means of an electron beam having an adequate intensity and of initial energy E.sub.i. On penetrating the sample, the electrons are decelerated and have a certain probability of passing from their initial energy E.sub.i to final energy states E.sub.f below E.sub.i. Generally, the passage between these two energy levels is accompanied by the emission of photons, whose energy hv.sub.if is such that hv.sub.if =E.sub.i -E.sub.f.
As a first approximation, the number of photons emitted having the energy hv.sub.if is proportional to the density of the unoccupied electron states n(E.sub.f) dependent on the energy level E.sub.f. A scan in initial energy E.sub.i of the incident electrons makes it possible by collecting, using appropriate systems, a fixed energy photon to determine the density of the electron states n(g).
The study of fixed energy photons, i.e. having the same wavelength, coming from a sample bombarded with an electron beam is known as "Bremstrahlung Isochromat Spectroscopy" or by the abbreviation "BIS", i.e. braking radiation isochromatic spectroscopy. It can also be called "inverse photoemission".
The hitherto known apparatus make it possible to detect either high energy photons, i.e. the energy of the photons is above 0.5 keV, or low energy photons, i.e. the energy of the photons is below 10 eV.
One apparatus which detects high energy photons is described in the article entitled "Densities of Unfilled One-Electron Levels in the Elements Vanadium and Iron through Zinc by Means of X-Ray Continuum Isochromats", which appeared in the Journal Physical Review B, 3rd series, Vol. 7, No. 8, pp. 3411 to 3419. This article describes an apparatus enabling the detection of photons which have an energy equal to 0.53 keV of an X-radiation by means of a fixed X-monochromator. The use of an X-monochromator, which is difficult to regulate, and the fact that the apparatus has a large size make is far from easy to use.
An apparatus detecting low energy photons is described in an article entitled "A VUV Isochromat Spectrometer for Surface Analysis", published in Applied Physics, 18, 1979, pp. 375 to 380. This article describes an apparatus enabling the detection only of photons with an energy equal to 9.7 eV of ultraviolet radiation by means of a Geiger-Muller counter placed behind a lithium fluoride window.
The main disadvantage of these two apparatus, as well as all other known apparatus, is that it is only possible to detect photons with a clearly defined energy level, i.e. they only operate at a single wavelength.